.bp
.NH 1
Inline substitution
.NH 2
Introduction
.PP
The Inline Substitution technique (IL)
tries to decrease the overhead associated
with procedure calls (invocations).
During a procedure call, several actions
must be undertaken to set up the right
environment for the called procedure.
.[
johnson calling sequence
.]
On return from the procedure, most of these
effects must be undone.
This entire process introduces significant
costs in execution time as well as
in object code size.
.PP
The inline substitution technique replaces
some of the calls by the modified body of
the called procedure, hence eliminating
the overhead.
Furthermore, as the calling and called procedure
are now integrated, they can be optimized
together, using other techniques of the optimizer.
This often leads to extra opportunities for
optimization
.[
ball predicting effects
.]
.[
carter code generation cacm
.]
.[
scheifler inline cacm
.]
.PP
An inline substitution of a call to a procedure P increases
the size of the program, unless P is very small or P is
called only once.
In the latter case, P can be eliminated.
In practice, procedures that are called only once occur
quite frequently, due to the
introduction of structured programming.
(Carter
.[
carter umi ann arbor
.]
states that almost 50% of the Pascal procedures
he analyzed were called just once).
.PP
Scheifler
.[
scheifler inline cacm
.]
has a more general view of inline substitution.
In his model, the program under consideration is
allowed to grow by a certain amount,
i.e. code size is sacrificed to speed up the program.
The above two cases are just special cases of
his model, obtained by setting the size-change to
(approximately) zero.
He formulates the substitution problem as follows:
.IP
"Given a program, a subset of all invocations,
a maximum program size, and a maximum procedure size,
find a sequence of substitutions that minimizes
the expected execution time."
.LP
Scheifler shows that this problem is NP-complete
.[~[
aho hopcroft ullman analysis algorithms
.], chapter 10]
by reduction to the Knapsack Problem.
Heuristics will have to be used to find a near-optimal
solution.
.PP
In the following chapters we will extend
Scheifler's view and adapt it to the EM Global Optimizer.
We will first describe the transformations that have
to be applied to the EM text when a call is substituted
in line.
Next we will examine in which cases inline substitution
is not possible or desirable.
Heuristics will be developed for
chosing a good sequence of substitutions.
These heuristics make no demand on the user
(such as making profiles
.[
scheifler inline cacm
.]
or giving pragmats
.[~[
ichbiah ada military standard
.], section 6.3.2]),
although the model could easily be extended
to use such information.
Finally, we will discuss the implementation
of the IL phase of the optimizer.
.PP
We will often use the term inline expansion
as a synonym of inline substitution.
.sp 0
The inverse technique of procedure abstraction
(automatic subroutine generation)
.[
shaffer subroutine generation
.]
will not be discussed in this report.